Allow marking options as ignored.

* jnlib/argparse.h (ARGPARSE_OPT_IGNORE): New.
(ARGPARSE_TYPE_MASK): New, for internal use.
(ARGPARSE_ignore): New.
* jnlib/argparse.c (optfile_parse, arg_parse): Replace remaining
constants by macros.
(optfile_parse): Implement ARGPARSE_OPT_IGNORE.
(arg_parse): Exclide ignore options from --dump-options.

In addition to the ignore-invalid-option (commit 41d56433) it is often
useful to mark options in a configuration which as NOP. For example
options which have no more function at all but can be expected to be
found in existing conf files. Such an option (or command) may now be
given as

ARGPARSE_ignore (300, "obsolete-option")

The 300 is merely used as a non-valid single option name much like
group names or the 500+n values used for long options.

(cherry picked from commit 54c54e2824aab5716a187bbbf6dff8860d6a6056)

Resolved conflicts:
common/argparse.c: Fixed.

• Signed-off-by: Werner Koch <wk@gnupg.org>